It is customary in connection with automotive vehicles using internal combustion engines to employ a so-called vacuum advance in which the ignition is advanced through a vacuum motor operatively connected to the distributor and to the intake manifold. Thus, as the intake manifold suction increases, the timing is advanced. When the engine is operating at normal driving speeds, the arrangement just described results in relatively efficient combustion with acceptable emission standards as far as pollution is concerned. When, however, the engine is idling or decelerating to a stop, the hydrocarbon output becomes relatively high percentagewise. Thus, when the automobiles are coming to a halt after being braked or when they are being driven at very low speeds in heavy traffic or are idling while waiting for a stop light to change, the hydrocarbon output is relatively high where the spark is advanced in the usual manner. It has been known for some time that retarding the spark under these conditions increases the extent to which the hydrocarbons are burned. This is partly due to the fact that the engine tends to operate at a higher temperature with a less advanced spark so that the resulting heat tends to burn some of the hydrocarbons in the exhaust with the result of a lower total output.
The more recent automobiles, beginning with the year 1968, are equipped with control devices to reduce pollution from exhaust emissions. These have taken several forms. The older automobiles, however, are particularly subject to the problem pointed out above. Consequently, various expedients have been proposed for retarding the spark under idling conditions and unless and until the temperature of the engine rises excessively; when the latter occurs, the means for interrupting the normal spark advance is overridden so as to restore the normal advance until the engine temperature drops.
The prior arrangements, for the most part, required certain modifications of the engine. It is highly desirable, if any number of vehicles are to be modified, that the means for modification of the equipment be of such nature that it can be very quickly attached to the automotive vehicle with a minimum of tools and with no appreciable modification of the automotive engine.